With the development of society and the continuous progress of science and technology, whether in people's lives or in enterprise production, the use of various electrical equipment has been increased, so that more power is needed to ensure the operation of these equipment. In China, although the power industry is constantly developing hydropower or clean energy, coal-fired units still account for a very large proportion. It is of great significance to strengthen the research and application of desulfurization wastewater treatment technology for coal-fired power plants, which can better reduce the impact of coal-fired power plants on the environment.
1 Water quality characteristics and influencing factors of desulfurization wastewater
1.1. Water quality characteristics of desulfurization wastewater
The composition and concentration of desulfurization wastewater have a great impact on the operation and management of the treatment system, and are the key factors affecting the selection of treatment equipment and corrosion. Desulfurization wastewater generally has the following characteristics.
1) The water quality is weakly acidic: the pH value varies from 5 to 50～6.5, 4 domestic0～6.0。
2) The content of suspended solids is high, and its mass concentration can reach tens of thousands of mg L.
3) COD, fluoride, heavy metals exceed the standard, including Class 1 pollutants, such as AS, HG, PB, etc.
4) High salt content, containing a large amount of SO42, SO32, Cl plasma, of which the mass fraction of Cl is about 004。
1.2 Factors affecting the water quality of desulfurization wastewater
The quality and quantity of desulfurization wastewater are mainly affected by the quality of coal, limestone, the design and operation of the desulfurization system, the pollutant control equipment in front of the desulfurization tower and the dewatering equipment.
1) Coal is the main pollutant of desulfurization wastewater, and the different types of coal will affect the discharge of desulfurization wastewater: the combustion of high-sulfur coal will produce more sulfur dioxide, which will increase the amount of desulfurizer and increase the discharge of desulfurization wastewater;In order to prevent the corrosion of the desulfurization system, maintain the chloride ion concentration in the desulfurization slurry at a certain level, which will increase the elimination of the desulfurization slurry and increase the discharge of desulfurization wastewater.
2) Some of the pollutants in the desulfurization wastewater are limestone, and the clay impurities in the limestone contain inert fine particles, aluminum and silicon. At the same time, limestone is important for nickel and zinc in desulfurization wastewater.
3) The influence of the design and operation of the desulfurization system on the water quality of the desulfurization wastewater is mainly reflected in the use of additives, oxidation mode or degree of oxidation, and the construction materials of the desulfurization system. Studies have shown that the use of acid additives has a high contribution rate to BOD5 in desulfurization wastewaterThe oxidation mode or oxidation degree has an important impact on the form of pollutants in the desulfurization wastewater, in the case of forced oxidation system or sufficient oxidation, the selenium in the desulfurization wastewater exists in the form of selenate, and in the case of non-forced oxidation system or insufficient oxidation, selenium exists in the form of selenite, the toxicity of se( ) is greater than that of se( ), but se( ) can be removed by co-precipitation of iron, and se( ) is not easy to remove, and can only be removed by biological treatment methods. The corrosion-resistant material can withstand the corrosion of higher concentrations of chloride ions in the slurry, which can increase the number of cycles of the desulfurization slurry and reduce the discharge of desulfurization wastewater.
4) The influence of pollutant control equipment before the desulfurization tower mainly refers to the impact of dust removal equipment and denitrification equipment. At present, there is no data showing that the increase in dust removal efficiency can significantly improve the water quality of desulfurization wastewater: when the dust removal efficiency of the electrostatic precipitator is increased from 998% to 99At 9%, theoretically, the concentration of total suspended particulate matter in the desulfurization wastewater will decrease slightly, but the fine particles of fly ash may increase the content of volatile metals in the desulfurization wastewater, so the increase in dust removal efficiency may have an important impact on the content of some metals in the desulfurization wastewater.
2 Analysis of current desulfurization wastewater treatment technology
2.1 Pre-treatment process
The pretreatment process of wastewater is divided into three steps:
(1) Chemical precipitation.
Wastewater contains a large amount of calcium ions and magnesium ions, and if you want to remove these two ions, you can add chemicals containing carbonate or hydroxide ions to form a precipitate between calcium ions and magnesium ions to reduce the hardness of wastewater. Commonly used agents are sodium carbonate and sodium hydroxide, both of which can form a precipitate, compared with sodium carbonate, sodium hydroxide has a wide range of drugs, less dosage and higher removal effect. In addition, according to the checked data, the calcium ions and magnesium ions in the wastewater can react with the carbon oxides and water vapor in the flue gas to produce calcium carbonate and magnesium carbonate substances, this technology makes full use of the internal conditions of the system, does not need to add any external substances, and effectively saves economic costs.
(2) Coagulation and precipitation. The purpose of coagulation sedimentation is to further remove impurities from the wastewater. After the first step of pretreatment, the wastewater still contains suspended solids. Therefore, in the second step, PAM or PAC is added, and the agitator is used to quickly stir the agent and wastewater at a certain speed, and the two are fully fused to form a flocculated substance similar to a snowflake. There are still some disadvantages of this method, although the removal effect is good, it is still difficult for the lightweight suspended solids to have a sedimentation effect. In addition, the stirring speed is a problem, too fast will form a flocculated substance earlier, which is easy to breakIt is too slow to precipitate.
(3) Filtration. The purpose of filtration is to further separate the solid and liquid to obtain a clearer water quality. Industrial wastewater generally uses filter or membrane filtration technology. The filter is an automatic filtration technology that simply feeds the treated industrial wastewater into the filter, but the effect of the filter is mediocre. Membrane filtration is a new type of technology, which has the advantage of good filtration effect, and the pores of the membrane are very small, and the filtration of small suspended solids occupies an advantageThe disadvantage is that the material needs to be changed regularly.
2.2 Advanced treatment process
2.2.1 Biological treatment. Biological treatment is the use of microorganisms to treat biodegradable soluble organic pollutants or to convert many insoluble pollutants into flocculents. The removal of pollutants can be done in three ways: aerobic, anaerobic or anoxic. Generally, power plants use aerobic methods to remove BOD5, remove metals or nutrients by anaerobic or anoxic methods, and microorganisms can reduce selenate or selenite to elemental selenium through respiration, which is adsorbed on the surface of microbial cells.
Biological treatment can effectively remove heavy metal elements such as selenium (down to G L level) and mercury (down to ng l level) in desulfurization wastewater, but its system is complex, expensive and easy to form toxic organic selenium and organic mercury, causing secondary pollution.
2.2.2 Hybrid zero-valent iron technology (HZVI).
It has been found that the use of ZVI can effectively reduce the content of selenate or selenite in wastewater, but with the progress of the reaction, the iron surface is easy to passivate, which affects the reactivity of ZVI. Later, some scholars introduced Fe2+ into the ZVI treatment system, and found that the reactivity of ZVI was significantly improved. Experimental studies have shown that the mercury removal efficiency of the mixed ZVI technology reaches 9999% (effluent concentration< 0005 g l), selenium removal efficiency up to 998% (effluent concentration< 7 g l). In addition, it has lower operating costs compared to technologies such as biological treatment.
2.3 Zero-emission treatment technology
In the current stage of desulfurization wastewater treatment technology, zero discharge treatment technology is the most advanced treatment technology, in this technology, according to the different treatment methods, it can be divided into two types: desulfurization wastewater and fly ash mixing technology, when using this technology to treat wastewater, it is to transfer the pollutant substances in the wastewater, so that the wastewater enters the fly ash, so that when the fly ash is transported, the dust will be inhibited. Evaporation pond treatment technology, the use of this technology for wastewater treatment, is the use of high temperature, the waste.
The water in the water evaporates, reducing the amount of wastewater. At this time, it needs to be treated in a corresponding way, using the transpiration of plants, typing spray evaporation equipment, etc., so that the treatment effect will be better, and the cost of investment is not very high, so it is loved by many coal-fired power plant owners. However, when using these speed-up methods, they often have a small impact on the environment, and further research is needed.
Wrapping up
To sum up, in the process of China's current social development, although coal-fired power plants have played an important role in it, it has provided sufficient power energy for the society. However, in the process of its operation, some desulfurization wastewater will often be produced, which will have a serious impact on the environment and is not conducive to the construction of sustainable development in China. Sulfur oxides are the key harmful substances that cause ecological pollution in China, in today, although there is still a distance to achieve zero discharge technology for industrial desulfurization wastewater technology, but it is believed that in the future tomorrow, coal-fired power plants can effectively control the production of sulfur oxides, and the desulfurization wastewater is evaporated and concentrated to achieve zero discharge, and give us a clean blue sky.
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